Dispersion compensating module and mode converter, coupler and dispersion compensating optical waveguide therein

ABSTRACT

A dispersion compensating module, mode converter, coupler and dispersion compensated optical fiber therein. The dispersion compensating fiber has a plurality of core segments, the refractive index profile being selected to exhibit properties such that an LP 02  mode at 1550 nm may be propagated a distance (generally 0.5-3.0 km), upon conversion to LP 02 , to compensate for dispersion of a length of transmission waveguide preferably greater than 25 km propagating in an LP 01  mode. In another embodiment, the dispersion compensating module has a mode converter having a reflective fiber grating for converting a first to a second mode interconnected to a dispersion compensated fiber propagating in the second mode. The mode converter has a coupler adapted to operatively couple light propagating in a first mode from a first fiber into a second, and a reflective fiber grating operatively coupled to the second fiber; the grating being capable of converting light from the first into the second mode. According to another embodiment, an optical fiber coupler is provided having a first fiber with a first propagation constant in a first mode, and a second fiber within the coupler having a second propagation constant, the second fiber including a necked-down portion which is formed prior to fusion of the fibers, the necked-down portion being formed such that the local propagation constant of the second fiber substantially matches the first propagation constant thereby enhancing first mode coupling.

[0001] This application claims priority to and the benefit of U.S.Provisional Patent Application No. 60/180,824, filed Feb. 7, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to an optical waveguide fiber and opticalcomponents. More particularly, the invention relates to a dispersioncompensating module and a mode converter, coupler and dispersioncompensating optical waveguide fiber useable therewith.

BACKGROUND OF THE INVENTION

[0003] Dispersion compensating fibers used in telecommunications systems10, such as illustrated in FIG. 1, correct for the unwanted effects ofdispersion of the transmission fiber 12. Transmission fibers 12preferably have a large effective area (e.g., >60 μm², and morepreferably greater than 70) and propagate light signals in thefundamental mode (LP₀₁). An example of a transmission fiber is LEAF®optical fiber manufactured by Corning Incorporated of Corning, N.Y.,which is designed to operate primarily at about the 1550 nm operatingwindow. In some systems, compensation takes place within a module 11having a length of Dispersion Compensating (DC) fiber housed within it.A section 13 of transmission fiber terminates at the module 11 and iscoupled with the DC fiber. After being dispersion compensated, the DCfiber is again coupled with the transmission fiber 12 and the signalcontinues along a continuing portion 14 of the transmission system 10.FIG. 1 illustrates a simple system deployment. It should be understoodthat typical transmissions systems include other devices such asamplifiers before and after the module, add/drop devices, etc.

[0004] One solution described in U.S. Pat. No. 5,185,827 and shown inFIG. 2, compensates for the dispersion of the transmission fiber byproviding a dispersive waveguide element which transmits the lightsignal at a higher-order LP₁₁ mode. An optical mode converter isutilized to convert the incoming signal from the fundamental modecarried by the transmission fiber to the higher-order mode LP₁₁, modethat is supported and carried by the dispersive waveguide element.Similarly, once the dispersion compensation is achieved, a secondoptical mode converter converts the light signal back to the fundamentalmode (LP₀₁). However, transmission in the LP₁₁ mode has a problem thatthe signal may be split into multiple modes due to slight imperfectionsin the fiber's circular geometry. This has the effect of undesirablydistorting the transmitted signal.

[0005] Thus, it should be recognized that the properties of the DC fiberused in the dispersion compensating module are vitally important to theperformance of the overall optical transmission system.

SUMMARY OF THE INVENTION

[0006] According to a first embodiment of the invention, an opticalwaveguide fiber suitable for use as a dispersion compensating fiber isprovided with improved properties such that it may advantageouslysupport light propagation in an LP₀₂ mode. Preferably, propagation issupported at a wavelength of about 1550 nm and for a sufficient distanceto compensate for dispersion of another fiber, for example an opticaltransmission fiber.

[0007] According to another embodiment of the invention, a DispersionCompensating (DC) waveguide fiber is provided comprising a plurality ofcore segments. The refractive index profile of the DC fiber is selectedto exhibit properties such that an LP₀₂ mode is supported and propagatedthereby at a wavelength of about 1550 nm. Upon conversion to the LP₀₂mode, preferably by an all-fiber mode converter according to anotherembodiment of the invention, the incoming signal is propagated withinthe DC fiber for an appropriate length (generally about 0.5-3.0 km,depending on the transmission fiber used). The DC fiber is designed tocompensate LP₀₂ mode for the dispersion effects of the transmissionoptical waveguide (the primary fiber transmitting in an LP₀₁ mode).

[0008] Preferably, the transmission waveguide, for which dispersioncorrection is occurring, has a length greater than 25 km, and moretypically on the order of between about 50 km-100 km. The inventiondescribed herein advantageously allows for a very short segment of DCfiber to accomplish the dispersion compensation. For example, in oneembodiment, less than {fraction (1/100)}^(th) of the length of thetransmission fiber may be required for compensation of certaintransmission fibers, for example Corning's LEAF® optical fiber.

[0009] In accordance with another aspect of the invention, the DCoptical waveguide fiber exhibits a kappa value between about 10 nm andabout 500 nm; where kappa is the ratio of dispersion in the LP₀₂ mode atabout 1550 nm divided by the dispersion slope in the LP₀₂ mode at about1550 nm. In accordance with a more preferred embodiment, the kappa valueis in the range between about 30 nm and about 70 nm. According toanother embodiment, the DC waveguide preferably has an effective areagreater than about 30 μm² at about 1550 nm, more preferably greater thanabout 60 μm², and more preferably yet between about 30 μm² and 150 μm²,and most preferably between about 50 μm² and about 90 μm².

[0010] In a preferred embodiment of the invention, the fiber comprises aplurality of, preferably at least three core segments. Preferably, firstand third segments of the plurality of segments comprise a dopant suchas germanium to raise the index of refraction of the core a sufficientamount with respect to the cladding to achieve the desired Δ%.Alternatively, any other suitable dopants such as phosphorous may beemployed. Moreover, fluorine doping may be employed to lower therefractive index of a second core region and/or the clad region ascompared to the core.

[0011] The geometry of the refractive index profile of the DC fiber isselected accordingly to enable transmission of the LP₀₂ mode oversubstantial distances (e.g., >0.5 km). For example, the structure, i.e.,the radius of the various segments, their width dimensions, and their Δ%values are selected in accordance with the invention as described in theseveral examples provided herein.

[0012] In accordance with one preferred embodiment, the waveguidecomprises a structure with:

[0013] (a) a first core segment having an outer radius in the rangebetween about 3 μm and 8 μm and a Δ% peak in the range between about1.0% and 2.5%,

[0014] (b) a second core segment having an outer radius in the rangebetween about 7 μm and 13 μm and a Δ% peak in the range between about0.3% and −0.5%, and

[0015] (c) a third core segment having an outer radius between about 10μm and 20 μm and a Δ% peak in the range between about 0.2% and 1.0%.

[0016] Other embodiments and more preferred values of radii, Δ% orcombinations thereof are described more fully in the specification andappended claims. Fibers with these ranges of radii and Δ% enabletransmission in the LP₀₂ mode.

[0017] In accordance with another preferred embodiment, the waveguidefiber comprises:

[0018] (a) an effective area in the range between about 50 μm²and 90 μm²at about 1550 nm and in the LP₀₂ mode,

[0019] (b) a dispersion value at about 1550 nm and in the LP₀₂ modebetween about −50 and −400 ps/nm/km, and

[0020] (c) a dispersion slope value at about 1550 nm and in the LP₀₂mode between about −0.01 and −20 ps/nm²/km.

[0021] Other preferred values of effective area, dispersion, dispersionslope, kappa or combinations thereof are more fully described in thespecification and appended claims.

[0022] According to another embodiment of the invention, a dispersioncompensating optical waveguide includes a plurality of core segments,the refractive index profile of which is selected to exhibit aneffective area between about 30 μm² and 150 μm² wherein the dispersioncompensating optical waveguide is capable of propagating light in theLP₀₂ mode a sufficient distance at about 1550 nm, upon being convertedfrom an LP₀₁ mode, to be capable of compensating for dispersion of alength of fiber transmitting in the LP₀₁ mode. Preferably, the fibertransmitting in the LP₁₀, mode is a long-haul waveguide having a lengthgreater than about 25 km. More preferably, the transmission fiber may bea fiber, such as LEAF® optical fiber available from CorningIncorporated, that exhibits an effective area greater than about 65 μM²in the LP₀₁ mode. Preferably, the DC optical waveguide has a lengthbetween about 0.5 km and about 3 km, thus providing a segment that isshort enough to conveniently package within a compact dispersioncompensating module.

[0023] In accordance with another embodiment of the invention, adispersion compensating module is provided including a reflective fibergrating to convert light propagating in a first mode into lightpropagating in a second mode. Most preferably, the module comprises acoupler adapted to couple a first fiber that is adapted to propagatelight in a first mode with a second fiber. In accordance with thisaspect of the invention, a reflective fiber grating is operativelyconnected to the coupler; the fiber grating being adapted to convertlight propagating in the first mode into a second mode. In thecompensating module in accordance with another aspect thereof, thesecond fiber is operationally and optically coupleable through thecoupler to the reflective fiber grating and the second fiber maypropagate light in a second mode. According to a preferred embodiment ofthe invention, the first fiber is a transmission fiber and the secondfiber is a dispersion compensating fiber. Preferably, the first mode isan LP₀₁ mode and the second mode is an LP₀₂ mode.

[0024] In accordance with a preferred embodiment, the dispersioncompensating module comprises a mode converter and a dispersioncompensating fiber. The mode converter is operatively coupleable with atransmission waveguide; the transmission waveguide being adapted topropagate light in a first mode. Within the mode converter is areflective fiber grating capable of converting the first mode into asecond mode. A dispersion compensating fiber is operatively coupled tothe mode converter and the dispersion compensating fiber is adapted topropagate light in the second mode to compensate for dispersion of thetransmission fiber.

[0025] According to another embodiment of the invention, the dispersioncompensating module comprises a mode converter adapted for operativelycoupling with an optical transmission waveguide, the transmissionwaveguide propagating light in a first mode. The mode converter includesa reflective fiber grating that is adapted to convert the first modeinto a second mode. The module also includes a dispersion compensatingfiber, operatively coupled to the mode converter, adapted to propagatelight in the second mode. The module preferably also includes a coupleradapted to couple light propagating in the first mode into thereflective fiber grating and which is further adapted to couple lightpropagating in the second mode into the dispersion compensating fiber.

[0026] In accordance with another embodiment of the invention, anoptical mode converter is provided comprising an optical fiber coupleradapted to operatively couple light propagating in a first mode in afirst fiber into a second fiber, and a reflective fiber gratingoperatively coupled to the second fiber, the grating being capable ofconverting light propagating in a first mode into a second mode whereinthe second fiber extends from the optical fiber coupler and is adaptedto propagate light in the second mode. Preferably, the first fiber is afiber pigtail adapted to operatively couple to an optical transmissionwaveguide propagating light in an LP₀₁ mode. Most preferably, thereflective fiber grating converts the LP₀₁ mode into an LP₀₂ mode; thefiber grating being operatively coupled with the pigtail through, forexample, an optical fiber coupler.

[0027] In one embodiment, a fiber interconnect operatively couples thereflective fiber grating with a DC fiber; the DC fiber adapted topropagate light in the LP₀₂ mode. The reflective fiber gratingpreferably includes a plurality of longitudinally spaced portions thathave been exposed to UV radiation to vary those respectiveportions'refractive index. Preferably, the longitudinal spacing of theportions are spaced at intervals that vary by up to 3% from a beginningto an end of the reflective fiber grating. It should be recognized thata broader spacing variation may be utilized if a broader gratingbandwidth is desired. Various characteristics of the preferredconversion fiber upon which the fiber grating is written are describedherein. In one embodiment, the conversion fiber comprises boron,germanium and phosphorous doped silica.

[0028] According to another embodiment of the invention, an opticalfiber coupler is provided wherein the propagation constants (in aparticular mode) of a first and second fiber therein are matched bystretching a portion of one of the fibers prior to fusion thereof. Inmore detail, the coupler comprising a first optical fiber within thecoupler having a first propagation constant in a first mode, and asecond fiber within the coupler, the second fiber having a secondpropagation constant in an undeformed portion thereof and in the firstmode that is different than the first propagation constant, the secondfiber including a necked-down portion formed on a glass portion thereofwhich is formed prior to fusion of the fibers, the necked-down portionhaving a dimension such that a third propagation constant in thenecked-down portion substantially matches the first propagation constantwherein coupling of light between the fibers in the first mode isenhanced. Further details of the dispersion compensating module and themode converter, coupler and various fibers included therein are in theattached disclosure, claims and drawings to follow.

Definitions

[0029] The following definitions are in accord with common usage in theart.

[0030] The refractive index profile is a plot of the relationshipbetween refractive index and waveguide fiber radius. It is generallyprovided as a Δ% as defined below.

[0031] A segmented core is one that has at least a first and a secondwaveguide core segment positioned at a radial distance from thewaveguide centerline. Each segment has a respective refractive indexprofile.

[0032] The radii of the segments of the core are defined in terms of thebeginning and end points of the segments of the refractive indexprofile. FIG. 5, for example, illustrates the definitions of radii R1,R2 and R3 used herein. The radius R1 of the first index segment 18, isthe length that extends from the waveguide centerline to the point atwhich the profile, when extrapolated with a tangential line, intersectsthe innermost portion of a tangentially extrapolated portion of the nextadjacent segment. The outer radius R2 of second segment 19 extends fromthe centerline to an outermost radial point of the second segment atwhich the tangentially extrapolated edge portion of the inner radius ofthe third core segment intersects the outermost point of the secondsegment. The outer radius R3 of third segment 20 extends from thecenterline to the radius point at which the descending tangentialportion of the third core segment intersects the zero Δ%, if forexample, there are additional segments utilized. The width of eachsegment 18, 19, and 20 respectively is measured with respect to theradii R1, R2-R1, and R3-R2, respectively.

[0033] The effective area is defined herein as:

[0034] A_(eff)=2Π (∫E² r dr)²/(∫E⁴ r dr), where the integration limitsare 0 to ∞, and E is the electric field associated with the mode inwhich the light is propagated and r is the radius within the integratedinterval.

[0035] The term Δ% represents a relative measure of refractive indexdefined by the equation:

Δ%=100×(n _(i) ² −n _(c) ²)/(2n _(i) ²)

[0036] where n_(i) is the refractive index in any region i along theprofile, and n_(c) is the refractive index of the cladding region,unless otherwise specified.

[0037] It is an advantage of the present invention that the DC waveguidefiber has greater effective area than prior DC fibers, thus providinglower nonlinear effects. This higher effective area is achieved by lighttransmission in the LP₀₂ mode. This has the advantageous effect ofreducing nonlinearities in the signal transmission.

[0038] It is another advantage of the present invention that the DCwaveguide fiber propagates light signals in the higher order LP₀₂ modeenabling high negative dispersion and negative slopes and therebyallowing compensation with shorter lengths of DC fiber. For example, ina preferred embodiment for use with LEAF® optical fiber, the length ofDC fiber required may be less than {fraction (1/100)}^(th) of thetransmission fiber's length. This enables shorter DC fiber lengths andthus lower losses as well as smaller DC modules. In particular, becausethe LP₀₂ transmission mode exhibits circular symmetry (an even symmetrymode), it is desirably very tolerant of circularity variations in thefiber. The present invention dispersion compensating fiber enables theiruse in such devices over a wide range of wavelengths (larger bandwidth)and with low attenuation.

[0039] Therefore, the present invention solves the problem of modesplitting when transmission is propagated in the prior art LP₁₁ mode.

[0040] An advantage of another embodiment of the invention is that themode conversion and dispersion compensation is accomplished with an allfiber based approach, thus enabling compact, robust and cost effectivemode conversion and dispersion compensation.

[0041] Other aspects and advantages of the invention will be understoodwith reference to the following detailed description, claims andappended drawings.

BRIEF DESCRIPTION OF THE FIGURES

[0042]FIG. 1 illustrates a block diagram of a portion of an opticaltransmission system of the prior art with which the present inventionhas utility.

[0043]FIG. 2 is a diagram illustrating the interconnection of adispersive waveguide element with an optical mode converter according tothe prior art.

[0044]FIG. 3 is a block diagram of a dispersion compensating module inaccordance with the present invention.

[0045]FIG. 4 is a perspective view of a portion of the dispersioncompensating optical fiber according to the invention illustrating coreand clad segments.

[0046] FIGS. 5-11 are graphs illustrating various index profilesplotting Δ% vs. core radius of several DC optical waveguides made inaccordance with the present invention.

[0047]FIG. 12 is a block diagram of a dispersion compensating moduleincluding the mode converter, coupler, conversion fiber and dispersioncompensating fiber in accordance with the present invention.

[0048]FIG. 13 is a partially sectioned side view illustrating the modeconverter in accordance with an aspect of the present invention.

[0049]FIG. 14 is a partially sectioned side view illustrating variousapparatus used to manufacture the mode converter in accordance with thepresent invention.

[0050]FIG. 15 is a graph illustrating a refractive index profileplotting Δ% vs. core radius of a conversion fiber in accordance with anaspect of the present invention.

[0051]FIG. 16 is a side view illustrating a stripped and stretchedportion of a fiber utilized with the coupler in accordance with anaspect of the present invention.

[0052]FIG. 17 is a graph illustrating a refractive index profile plot ofΔ% vs. core radius of a second embodiment of a conversion fiber inaccordance with an aspect of the present invention.

[0053]FIG. 18 is a partially sectioned side view illustrating anotherembodiment of mode converter in accordance with another aspect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0054] The dispersion compensating optical waveguide 15 in accordancewith one aspect of the present invention is best illustrated withreference to FIGS. 3-11 herein. Referring first to FIG. 3, is shown aportion of a high data rate telecommunication system 10′ including aprimary portion of transmission fiber 13, such as LEAF® optical fiberavailable from Corning Incorporated, which exhibits a positive chromaticdispersion and a positive dispersion slope. The transmission fiberterminates, and a new transmission portion 14 starts, at a dispersioncompensating module 11′. The portion 13 prior to the module 11′ is of asufficient length such that the fiber's dispersion properties havedistorted the signal to the point that dispersion compensation isdesirable. The length of the portion 13 may be, for example, greaterthan about 25 km and, more preferably, on the order of about 50 km-100km or more. The system may, for example, include many dispersioncompensating modules 11′ at various positions along any particulartransmission segment.

[0055] In particular, within the dispersion compensating module 11′, asbest illustrated in FIG. 3, a mode converter 16 a converts a first mode,such as the fundamental LP₀₁, mode, transmitted in and propagated by thefirst segment 13 of transmission fiber to a second, higher-order LP₀₂mode. The signal is then propagated in the LP₀₂ mode in the dispersioncompensated fiber 15 a sufficient distance, i.e., through an appropriatelength of DC fiber 15, to partially or completely compensate for thedispersion caused by the primary transmission fiber portion 13. Thefiber 15 may be wound about a spool or cylinder 17 or other like holderpreferably mounted to the module 11 to enable simple and compact moduleconstruction. Advantageously, by propagating in the LP₀₂ mode, largedispersion slope compensation is possible and smaller lengths of the DCfiber 15 are required to accomplish dispersion compensation. Typically,0.5 km to 3.0 km of the DC fiber 15 are desired to accomplish thedispersion compensation when an LP₀₂ mode is used for compensating forthe dispersion effects of LEAF® optical fiber, for example.

[0056] Preforms for such DC optical fibers 15 may be made using any ofthe known methods in the art, including chemical vapor depositiontechniques such as OVD, PCVD, MCVD and VAD. In a preferred embodiment, asoot preform is manufactured using an OVD technique having the desiredrefractive index profile in accordance with the present invention. Thissoot preform is then consolidated in a consolidation furnace and drawninto a DC waveguide 15 as is well understood by those of skill in theart. According to a preferred aspect, the core portion, i.e., thatportion carrying most of the light signal and defined by the profilesherein, is prepared as a blank (otherwise referred to as a core cane),consolidated and then overclad with silica soot to form the resultantpreform which is again consolidated and drawn into the opticalwaveguide. It should be understood that the core profiles describedherein may be developed in a multiple step method where a first corecane is drawn, another core segment is deposited, consolidated, andagain drawn into core cane.

[0057] According to the invention, the DC optical fiber 15 includes thefollowing characteristics. First, the fiber preferably includes aplurality of core segments, preferably at least three, such as first,second, and third, radially-spaced segments, 18, 19, and 20,respectively, as best illustrated in FIG. 4. First, second, and thirdsegments 18, 19 and 20 generally make up the so-called physical core 26.The first core segment 18 has the shape of a nearly continuous lengthrod, whereas the second and third core segments 19, 20, respectively,have the shape of nearly continuous length cylinders surrounding the rodand having predetermined width dimensions of their radial walls. The DCwaveguide 15 according to the invention also includes a clad portion 24.This clad portion 24 is preferably pure silica and generally forms thebasis for determining Δ% as defined herein above. The core segments18,19, and 20 for one embodiment, as best shown in FIGS. 4 and 5, aregenerated at different steps during the formation of the preform by theaddition of various dopants in predetermined concentrations. Forexample, segments 18 and 20 preferably comprise germanium and silica.Segment 19 is preferably undoped such that it exhibits a Δ% ofsubstantially zero. Alternately, the segment 19 may comprise some smallpercentage of germanium. In an alternative embodiment, fluorine may beadded to down dope the second segment 19 slightly. The cladding 24preferably consists of pure silica. The DC fiber 15 achieves its desiredcharacteristics to enable the propagation of the LP₀₂ mode because ofits profile characteristics. In another aspect, the DC fiber 15 inaccordance with the invention is also capable of propagating in an LP₁₁mode.

[0058] In particular, according to a first aspect, the fiber includes aplurality of core segments, the refractive index profile of which isselected to exhibit properties such that it is capable of propagating asignal in an LP₀₂ mode. The signal is transmitted by the DC opticalwaveguide 15 a sufficient distance (through an appropriately selectedlength of fiber—preferably 0.5 km-3.0 km, upon conversion to the second,higher-order LP₀₂ mode, to compensate for dispersion of an incomingportion of a transmission optical waveguide 13 (FIG. 3). Typically, theportion 13 has a length greater than 25 km and propagates light in afirst, lower-order LP₀₁ mode.

[0059] According to one preferred aspect, the DC waveguide 15 exhibits akappa value K between about 10 nm and about 500 nm. Kappa K is definedas the ratio of dispersion in the LP₀₂ mode at 1550 nm divided bydispersion slope in the LP₀₂ mode at 1550 nm. More preferably, the valueK is in the range between about 30 nm and 70 nm.

[0060] According to another aspect of the invention, the DC waveguide 15comprises an effective area greater than about 30 μm², and morepreferably greater than about 60 μm² in the LP₀₂ mode and at about 1550nm. Preferably, the effective area is in the range between about 30 μm²and 150 μm², and more preferably yet between about 50 μm² and 90 μm² at1550 nm.

[0061] The DC fiber 15 preferably exhibits a dispersion value at 1550 nmand in the LP₀₂ mode of between about −10 and −1000 ps/nm/km, and morepreferably between about −50 and −400 ps/nm/km. The dispersion slopevalue of the DC fiber 15 at 1550 nm and in the LP₀₂ mode is preferablyless than −0.01, and more preferably between about −0.01 and −20ps/nm²/km, and more preferably yet between about −1.0 and −10 ps/nm²/km.

[0062] As illustrated in FIGS. 4 and 5, the DC waveguide 15 preferablyexhibits at least three core segments which preferably have thefollowing physical dimensions. The first core segment 18 has an outerradius dimension R1 in the range between about 3 μm and 9 μm, and morepreferably between about 4 μm and 8 μm. A second core segment 19preferably comprises a width dimension (R2-R1) in the range betweenabout 2 μm and 8 μm, and more preferably between about 4 μm and 6 μm.Preferably, the second core segment 19 has an outer radius R2 betweenabout 10 μm and 20 μm, and more preferably between about 7 μm and 13 μm.The third core segment 20 preferably includes a width dimension (R3-R2)in the range between about 1 μm and 10 μm, and more preferably betweenabout 4 μm and 8 μm. The outer radius R3 of the third segment ispreferably between about 10 μm and 25 μm, and more preferably betweenabout 12 μm and 18 μm.

[0063] According to a preferred embodiment of the invention the firstcore segment 18 comprises refractive index peak Δ% n₁ of greater thanabout 1.5%, and more preferably greater than 2.0%. The peak Δ% no of thefirst core segment includes a range between about 1.0% and 2.5%, andmore preferably between 1.5% and 2.5%. In the embodiment of FIG. 4, thefirst core segment 18 exhibits a Δ% of less than about 1.0 at thecenterline of the DC waveguide 15 and a Δ% peak (preferably greater than1.5%) at a radius location that is greater than 1 μm. The first coresegment 18 preferably has a peak Δ% preferably positioned at betweenabout 1 μm and 3 μm.

[0064] In a preferred embodiment of the invention, the second segment 19preferably exhibits a peak Δ% n₂ greater than zero. However, n₂ lessthan about 0.3% but preferably greater than or equal to −0.5%, and morepreferably less than 0.3% and greater than −0.1% will also provide thedesired properties. The third core segment 20 preferably comprises a Δ%n₃ in the range between about 0.2% and 1.0%, and more preferably between0.3% and 0.6%.

[0065] In all cases, it is preferable that the peak Δ% n₁ of a firstcore segment 18 be greater than the peak Δ% n₃ of the third core segment20. Furthermore, it is preferable that the peak Δ% n₃ of the thirdsegment be greater than the peak Δ% n₂ of the second core segment 19.Preferably, the peak Δ% of first, second, and third core segments areall greater than or equal to zero.

[0066] Table 1 below sets forth below a number of examples of DCwaveguide fibers 15 made in accordance with the present invention thathave properties enabling the transmission of a higher-order LP₀₂ mode,for example, within a dispersion compensating module. It should beunderstood that the examples that follow are illustrative only and thata wide variety of variants with similar characteristic to thosedescribed herein may achieve propagation of light signals in the LP₀₂mode within the DC waveguide fiber 15 such that dispersion compensationmay be achieved. TABLE 1 Dispersion (ps/nm/km) Effective at Area (μm²)at Example 1550 nm Kappa 1550 nm Number and LP₀₂ (nm) and LP₀₂ 1 −941 3494 2 −490 58 92 3 −109 48 69 4 −103 54 79 5 −125 53 64 6 −183 69 92 7−171 87 147

[0067] FIGS. 6-11 illustrate several additional profile plots of Δ%versus radius dimension for the above-listed example numbers. FIGS. 6,7, 8 and 9 relate to example numbers 1, 2, 4, and 5, respectively. FIGS.10 and 11 correspond to example numbers 6 and 7, respectively. FIG. 4corresponds to example number 3. Each of the profile plots 6-9 exhibitproperties as heretofore mentioned with reference to FIG. 4 such thatthey are capable of propagating light in the LP₀₂ mode.

[0068]FIG. 10 illustrates another embodiment (example 6) of DC fiber 115which exhibits a refractive index profile which is desirable forcompensating for dispersion of a transmission fiber 113 in atransmission system 110, for example, as is illustrated in FIG. 12. Thefiber 115 includes a first core segment 118 having an outer radius R1 inthe range between about 3 μm and 9 μm, more preferably between about 4μm and 8 μm, and most preferably about 6 μm. The first segmentpreferably includes a Δ% peak of greater than 1.5% and more preferablygreater than 2.0%. A second core segment 119 within the fiber has anouter radius R2 in the range between about 7 μm and 13 μm, and morepreferably between about 10 μm and 12 μm, and most preferably about 11μm. The second segment 119 preferably includes a Δ% peak in the rangebetween about 0.3% and −0.5% and more preferably between 0.3% and 0.1%,and most preferably about 0.2%. Preferably, a third core segment withinthe fiber 115 has an outer radius between about 10 μm and 25 μm, andmore preferably between 14 μm and 20 μm, and most preferably about 17μm. Preferably the third segment 120 includes a Δ% peak in the rangebetween about 0.2% and 0.8%, more preferably between 0.3% and 0.7%, andmost preferably between 0.5% and 0.6%. The fiber preferably includesgermania doped silica in segments 118, 119, and 120; the amounts beingvaried per each segment to achieve the various Δ%.

[0069]FIG. 11 illustrates a refractive index profile of anotherembodiment of dispersion compensating fiber 115 in accordance with thepresent invention that is particularly effective for use with the modeconverters 116 a, 116 b described with reference to FIG. 12. Thus, theprofile will be explained in detail with reference to both FIGS. 11 and12. As before described herein, the DC fiber 115 includes a profileincluding first 118, second 119 and third 120 core segments and a cladportion 124 encircling the last core segment. The preferred radii andΔ%'s of the segments 118, 119, 120 are as heretofore described withreference to FIG. 11. However, in this embodiment of fiber, the firstcore segment 118 preferably includes a sub-segment 125 with a lower Δ%.The first segment 118 preferably has an outer radius R1 in the rangebetween about 4 μm and 8 μm and a Δ% peak of greater than about 1.5%.The sub-segment 125 within the first core segment 118 has an innerradius Ri of between about 3 μm and 6 μm and a Δ% peak in the rangebetween about 0.6% and 1.4%, and more preferably between about 0.8 and1.2%. The step or sub-segment 125 in the first segment is provided suchthat the profile of the DC fiber 115 is better matched to the fiberinterconnects 126 which serves the function of interconnecting betweenthe couplers 128 and the DC fiber 115 or between the conversion fiber132 and the DC fiber. The fiber interconnect propagates light in theLP₀₂ mode to and from the DC fiber 115 and to and from the couplers andconversion fiber 132. Thus, the interconnect 126 serves an interconnectfunction by transmitting the light signal between the coupler and the DCfiber 115. Matching the profiles of the fiber interconnect and the DCfiber desirably lowers the losses and reduces mode coupling in thesplice (labeled X).

[0070] As best illustrated in FIGS. 12 and 13, the mode converters 116a, 116 b are preferably housed within an enclosure of the DC module 111.The packaging may be of any appropriate shape. Optionally, theconverters 116 a, 116 b may be separately packaged and interconnected tothe DC fiber and transmission fibers or other components by anyconvenient means. Each mode converter 116 a, 116 b preferably includes apigtail 130 which is spliced to or otherwise interconnected either to anamplifier section 121 or directly to the transmission fiber 113,114. Thepigtail 130 also interconnects to the DC fiber 115 by conventionalsplicing technology; the splices being designated as X's in FIG. 12. Inany event, the mode converters 116 a, 116 b are operatively connected tothe transmission fiber and also to the DC fiber 115. The operativeconnection to the transmission fiber is preferably through an amplifierstage 117. However, it should be recognized that the mode converter inaccordance with the invention is capable of use in a multitude ofapplications where conversion from a first mode into a second mode isdesired. The operative connection to the DC fiber 115 is through fiberinterconnects 126.

[0071] Now referring to FIG. 13 are shown the details of the modeconverters 116 a, 116 b in accordance with one embodiment of theinvention. It should be recognized that the mode converter may bepackaged in any appropriate manner and may be an unpackaged subassemblywithin the DC module 111. With reference to FIG. 13, the details of onemode converter 116 a will be described. It should be understood that themode converter 116 b is similar in construction; the differences incomparison thereto being only in the orientation of the reflective fibergrating. The converter 116 a includes a pigtail 130 adapted for splicingto operatively couple to another component, such as an amplifier stage(e.g., 121) or to incoming transmission fiber (e.g., 113). The pigtail130 may be manufactured of any suitable fiber. One preferable pigtailfiber is a single mode fiber, such as SMF-28™ optical fiber, availablefrom Corning Incorporated of Corning, N.Y. The pigtail 130 isinterconnected to an optical coupler 128. The coupler 128 also hasinterconnected to it a fiber interconnect 126. The fiber interconnect126 is a fiber which functions to operatively couple and interconnectthe coupler 128 to the DC fiber 115. This fiber interconnect 126 is alsooptically coupled to a converting fiber 132 that includes thereon areflective Bragg grating 134. The coupler 128 serves the purpose ofcoupling the light propagating in a first fiber, such as in the fiberpigtail 130 into one or more fibers, such as the converting fiber 132.

[0072] In this case, the coupler 128 operatively couples the light beingpropagated in a first mode, such as a fundamental or lower order mode,for example, the LP₀₁ mode, in the pigtail 130 into the converting fiber132 where it is converted by the reflective grating 134 into lightpropagating in an LP₀₂ mode. In the preferred embodiment of theconverter, the light signal is reflected back into the coupler 128 bythe reflective fiber Bragg grating 134 written onto the conversion fiber132. The coupler 128 then operatively couples the light signalpropagating in the LP₀₂ mode into the fiber interconnect 126. This fiberinterconnect 126 operatively couples and interconnects with the LP₀₂propagating DC fiber 115, preferably a DC fiber exhibiting refractiveindex profiles such as those described with reference to FIGS. 5-11.FIGS. 13 and 18 illustrate several embodiments of coupler assembliesillustrating the operative connections to the fiber interconnect 126 andthe conversion fiber 132. It should be understood that the interconnect126 and the conversion 132 fibers may be separate fibers that arespliced together as shown in FIG. 18 or the same fiber as shown in FIG.13. Preferably, upon traveling the appropriate distance through the DCfiber, some or all of the dispersion of the transmission fiber 113 iscompensated for.

[0073] In one embodiment, as best illustrated by FIG. 14, the coupler116 a is manufactured by inserting a pigtail 130, preferably asingle-mode fiber, into a tubular glass cane sleeve 136 of made up ofpreferably 4%-8% boron doped silica glass. The sleeve 136 preferably hasa length of about 70-72 mm, an inner diameter dimension of about 0.27mm, and an outer diameter dimension of about 2.6 mm. A pass throughfiber 131 made up of the fiber interconnect portion 126 is also insertedthrough the cane sleeve 136 and a stripped portion is appropriatelypositioned adjacent to the pigtail 130. The sleeve 136 is held bymoveable chuck members 142 a, 142 b that clamp onto each end of thesleeve 136 and that may be released and removed when desired. The fiber131 passing entirely through the sleeve 136 includes a short portion 137which has the protective polymer coating 133 stripped therefrom. Therevealed glass in that portion 137 is then heated by a longitudinallymoving burner and pulled under tension, preferably prior to insertioninto the sleeve 136 and prior to fusion of the fibers 130, 131, therebyforming a necked-down portion 138 of preferably approximately constantdimension within the necked-down portion. The necked-down portion 138 ispreferably between about 30% and about 60% of the original diameter ofthe glass portion of the fiber 131 and is preferably slightly shorterthan the length of the cane sleeve 136. In a preferred embodiment, thefiber interconnect 126 is the same profile as the conversion fiber 132and together make up the pass through fiber 131.

[0074] The amount of necking down required is determined based upon thepropagation constant β of the first fiber, e.g., the pigtail 130.Essentially, the second fiber 131 is precisely stretched under alongitudinally moving methane/oxygen flame until the propagationconstant β of the second fiber 131 in the necked down area 138 isapproximately matched with the propagation constant β of the first fiber130 fiber in the LP₀₁ mode and at 1550 nm. The stretching and neckingdown affects the core diameter which, in turn, affects the propagationconstant β. Matching the propagation constants in the LP₀₁ mode betweenthe two fibers 130,131 with different refractive index profiles improvesthe LP₀ mode coupling between the fiber thereby desirably minimizingcoupling losses.

[0075] The fibers 130, 131 are then appropriately positioned within thesleeve 136, the assembly including the end of fiber 130, the necked downportion 138 and the sleeve 136 are then locally heated by a burner 140,such as a methane and oxygen ring-type flame burner. Upon the assemblybeing heated, the sleeve 136 collapses onto the fibers 130 and 131 andfuses them together. While keeping the fibers and sleeve 136 above theglass transition temperature, the chuck members 142 a, 142 b areseparated while still holding the respective ends of the sleeve 136until the monitored coupling reaches a target dimension. This preferablytakes an increase in separation of between about 5 mm and 15 mm. Thechuck members 142 a, 142 b and heat are removed and the resultingcoupler 128 is formed as shown in FIG. 13 whereby the various fibers130, 131 become fused together at the mid-region of the coupler 128. Thefiber used for the fiber interconnect 126 and the converter fiber 132 inthis embodiment preferably exhibit a profile as shown in FIG. 15 or 17.Adhesive or other potting compound 139 is provided at the respectiveends to further secure the fibers 130, 131. It should be recognized thatthe present invention coupler may be employed anywhere fibers, such asfibers 126 and 130 have a mismatched propagation constant at aparticular wavelength. Moreover, although one method has been explainedfor achieving the necking down feature, other methods may be employed ifpracticable, such as etching with hydrofluoric acid solution.

[0076] For example, in FIG. 18, the fiber pigtail 330 couples into a DCfiber 315 which functions as the pass through fiber; such DC fiber beingpreferably identical to that described with reference to FIG. 10 or 11.The fiber 315 is spliced at splice “a” to a converter fiber 332 havingthe reflective fiber grating 334 as described herein written thereupon.The conversion fiber 332 preferably has the profile described withreference to either FIG. 15 or 17. The fiber interconnect 326 in thisembodiment is the DC fiber and at splice “b”, the DC fiber is preferablyspliced to an identical DC fiber 315′ mounted on any type of holder (notshown). The coupler 328 is manufactured as described above withreference to FIG. 14. Further details on manufacturing methods forcouplers may be found with reference to U.S. Pat. No. 5,295,211, whichis hereby incorporated by reference.

[0077] In FIG. 15, a profile of a first embodiment converter fiber 132is illustrated. This fiber may also be used as the fiber interconnect orthe pass through fiber. The fiber attaching to the DC fiber 115 in thiscase is the pass through fiber 131 and preferably exhibits a profile, atleast on an innermost core portion, which is preferably substantiallymatched in shape to the DC fiber 115. This minimizes the losses and modecoupling in propagating light at the splice between the two fibers.

[0078] The fiber 131 preferably includes a profile where R1 of the firstsegment 218 is in the range between about 3 μm and 8 μm, and mostpreferably about 5 μm. The first segment 218 preferably includes a firstΔ% peak of between about 1.4% and 2.5%, and more preferably betweenabout 1.8% and 1.4%. The second core segment 219 within the fiber has anouter radius in the range between about 6 μm and about 14 μm, morepreferably between about 6 μm and about 12 μm, and most preferably about8 μm. The second segment 219 preferably has a Δ% peak in the rangebetween about 0.4% and −0.5%, more preferably between about 0.4% andabout 0.2%, and most preferably about 0.3%. Preferably, a third coresegment 220 within the fiber 131 has an outer radius R3 between about 12μm and about 20 μm, more preferably between about 14 μm and about 18 μm,and most preferably about 16 μm. Preferably the third segment 220includes a Δ% peak in the range between about 0.6% and about 0.2%, morepreferably between about 0. 6% and about 0.3%, and most preferably about0.4%.

[0079] The outer sub-segment 225 within the first core segment 118preferably includes an inner radius Ri of between about 3 μm and about 6μm and a second Δ% peak (lower than the first Δ% peak) in the range ofbetween about 0.6% and 1.4%, and more preferably between about 0.8 andabout 1.2%. Optionally, the segments 219 and 220 of the fiber 131 maycomprise pure silica, and thus a Δ% of zero, as indicated by the dottedline segment labeled 144. The dotted segment 144 is shown slightly abovezero for illustration purposes only, but it should be recognized thatthe Δ% of pure silica segment would be exactly at zero Δ%.

[0080] Yet another embodiment of conversion fiber 132 is illustrated inFIG. 17. Again, this described fiber profile may be utilized as thefiber interconnect 126 or the pass through fiber 131. In thisembodiment, the profile consists of only segment one 318 with onesub-segment 325. The segment 318 preferably includes a profile where R1of the first segment 318 is in the range between about 3 μm and about 8μm, and most preferably about 5 μm. The first segment 218 preferablyincludes a Δ% peak of between about 1.4% and about 1.8%, and morepreferably between about 1.8% and about 1.4%. The outer sub-segment 325within the fiber has an inner radius Ri in the range between about 2 μmand about 6 μm, more preferably between about 2 μm and about 4 μm, andmost preferably about 3.5 μm. The sub-segment 325 preferably includes aΔ% peak of between about 0.6% and about 1.2%, and more preferablybetween about 0.9%. The inner sub-segment 321 preferably includes boronin the range of about 5%-15%, and most preferably about 11%, andgermanium in the range between about 25%-35%, and most preferably about30%.

[0081] In the FIG. 15 and 17 embodiments, the outer sub-segment 225, 325preferably includes phosphorous doped silica in about 15-25% by weight,and most preferably about 21% by weight.

[0082] To enable the ease of writing or imprinting the reflective fibergrating 134 onto the conversion fiber 232, 332, the inner region labeled221, 321 in the first segments 218, 318 of the profile, as illustratedin FIGS. 15 and 17, preferably both include a boron dopant present inabout 5%-15% by weight, and most preferably about 11% by weight. Theboron is added because of its photosensitivity-enhancing properties inthat the gratings may be provided on the innermost core 221, 321 of theconversion fiber 232, 332 by exposure of fiber to ultraviolet radiation.The second core segment 219 and the sub-portions 225, 325 preferablyinclude phosphorous. The sub-segments 225, 325 preferably includesphosphorous in a weight percentage of about 15%-25%, and most preferablyabout 21%. Second core segment 219 includes phosphorous in the amount ofabout 6% by weight and no germanium. Segment three 220 preferablyincludes 6% germanium by weight and no phosphorous. The addition ofphosphorous in these above-mentioned segments tends to retardphotosensitivity when exposed to UV radiation. Thus, the gratings are,for the most part, written onto the innermost part of the fiber's core,i.e., on sub-segment 221, 321.

[0083] The reflective gratings 134 are provided in accordance with theinvention by exposing a mask including transverse slots formed thereinto UV radiation having a wavelength of about 190 nm to about 270 nm. Thetransverse slots are oriented perpendicular to the length of the fiber132 and are positioned in front of the fiber 132 and in close proximitythereto. The exposure mask for writing the gratings preferably has slotwidths of about (0.5) microns and a nominal spacing (center-to-center)of about 1 microns. Notably, the nominal spacing does vary slightly (byup to 3%) from one end to the other. On the module 116 a, the spacing onthe left side of the grating is larger than the spacing on the righthand side by about 3%. This differential spacing enables conversion fromthe LP₀₁ mode to the LP₀₂ mode within the grating 134 over a broad bandof wavelength. Conversely, in module 116 b, the spacing betweenindividual grating regions is less when the grating is first encounteredand greater at the end. This converts LP₀₂ to LP₀₁ within the grating134 in converter 116 b.

[0084] Again referring to FIG. 12 and 13, in operation, a light signalpropagating in first lower-order mode, such as LP₀₁, in the transmissionfiber passes into the amplifier stage 117 and then into the pigtail 130on a first side of the dispersion compensating module 111 provided inthe telecommunication/data communication system 110. The light thenpasses through the coupler 128 where the light signal is coupled into asecond fiber, such as the conversion fiber 132. The conversion fiber hasa length on the order of 5 cm to several meters or more and includes aterminal end 142. The light, upon encountering the reflective fibergrating 134, is reflected back and simultaneously converted to a secondmode, such as the higher-order LP₀₂ mode. Little if any of the LP₀₂ modecan be propagated back into the pigtail 130 upon reflection because ofthe mismatch in the propagation constants β between the pigtail 130 andthe conversion fiber 132 in the LP₀₂ mode. Notably, the conversion fiber132 and the fiber interconnect 126, because of their refractive indexprofiles, as illustrated in FIGS. 15 and 17, are designed to readilypropagate light an LP₀₂ mode.

[0085] Upon exit from the converter 116 a, the light signal is thencarried into the dispersion compensating fiber 115 mounted on holder 117whereby compensation for the dispersion of the transmission fiber 113takes place. Preferably, the DC fiber described with reference to FIG.11 is utilized with the mode converters 116 a, 116 b described herein asthe fiber connect 126 and the DC fiber 115 have preferably matchedprofiles (at least for the innermost first segment) which leads to lesstransmission loss and mode coupling at the splices between the fiberinterconnect 126 and the DC fiber 115.

[0086] Upon exiting the DC fiber 115 wound on holder 117, the lightsignal propagates into fiber interconnect 126 of converter 116 b andthrough coupler 128 thereof where the light signal is coupled intoanother fiber 132 including another fiber grating 134 as heretoforedescribed with reference to 116 a (except that the spacing intervals arereversed).

[0087] Within the mode converter 116 b, the light signal is convertedback to a lower order LP₀₁, mode by the reflective fiber grating 134 andreflects back into the fiber pigtail 130. The pigtail readily propagateslight in the LP₀₁ mode because of the matched propagation coefficients.The signal then passes into the next amplifier stage 117 or into anothersection of transmission fiber or, optionally, directly into anopto-electronic detector or other optical module. Accordingly, it shouldbe recognized that upon passing through the DC module 111, thedispersion of the previous length 113 has been partially or fully,compensated for. Moreover, it should be recognized that a plurality ofalternating lengths of transmission fiber, amplifier stages 117, and DCmodules 111 including the dispersion compensating fiber 115, the modeconverter and coupler in accordance with the-present invention may beutilized in series to compensate for dispersion over any desired systemdistance. Moreover, it should be recognized that a second mode converter116 b may not be required in all instances.

[0088] It will be apparent to those of ordinary skill in the art thatvarious modifications and variations can be made to the presentinvention without departing from the scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations provided they come within the scope of the appended claimsand their equivalents.

What is claimed is:
 1. A dispersion compensating optical waveguidecomprising: a plurality of core segments, the refractive index profileof which are selected to result in an optical waveguide capable ofpropagating a light signal in an LP₀₂ mode a sufficient distance tocompensate for dispersion of a transmission optical waveguide having alength greater than 25 km that propagates the signal in an LP₀₁ mode. 2.The waveguide of claim 1 wherein the plurality of core segments furthercomprise: (a) a first core segment having an outer radius in the rangebetween about 3 μm and 9 μm and a Δ% peak in the range between about1.0% and 2.5%, (b) a second core segment having an outer radius in therange between about 7 μm and 13 μm and a Δ% peak in the range betweenabout 0.3% and −0.5%, and (c) a third core segment having an outerradius between about 10 μm and 25 μm and a Δ% peak in the range betweenabout 0.2% and 1.0%.
 3. The waveguide of claim 1 wherein the pluralityof core segments further comprise: (a) a first core segment having anouter radius in the range between about 3 μm and 9 μm and a Δ% peak inthe range between about 1.0% and 2.5%, (b) a second core segment havinga width in the range between about 2 μm and 8 μm and a Δ% peak in therange between about 0.2% and −0.5%, and (c) a third core segment havinga width in the range between about 1 μm and 10 μm and a Δ% peak in therange between about 0.2% and 1.0%.
 4. The waveguide of claim 1 whereinthe plurality of core segments further comprise: (a) a first coresegment having an outer radius in the range between about 4 μm and 8 μmand a Δ% peak in the range between about 1.5% and 2.5%, (b) a secondcore segment having a width in the range between about 4 μm and 6 μm anda Δ% peak in the range between about 0.3% and 0.1%, and (c) a third coresegment having a width in the range between about 4 μm and 8 μm and a Δ%peak in the range between about 0.3% and 0.7%.
 5. The waveguide of claim1 wherein said refractive index profile is selected to provide: (a) aneffective area greater than about 60 μm² at 1550 nm in the LP₀₂ mode,(b) a dispersion value at about 1550 nm in the LP₀₂ mode between about−50 and −400 ps/nm/km, and (c) a dispersion slope value at about 1550 nmin the LP₀₂ mode between about −0.01 and −20 ps/nm²/km.
 6. The waveguideof claim 1 wherein the plurality of core segments further comprise: (a)a first core segment having an outer radius in the range between about 3μm and 8 μm and a Δ% peak of greater than 1.5%, (b) a second coresegment having an outer radius in the range between about 8 μm and 12 μmand a Δ% peak in the range between about 0.3% and 0.1%, and (c) a thirdcore segment having an outer radius between about 14 μm and 20 μm and aΔ% peak in the range between about 0.2% and 0.6%.
 7. The waveguide ofclaim 1 wherein the plurality of core segments further comprise: (a) afirst core segment having an outer radius in the range between about 4μm and 8 μm and a Δ% peak of greater than about 1.5%, and (b) asub-segment within the first core segment having a inner radius ofbetween about 3 μm and 6 μm and a Δ% peak in the range between about0.6% and 1.4%.
 8. The waveguide of claim 1 wherein the refractive indexprofile is selected to result in the waveguide exhibiting a kappa valuebetween about 10 nm and about 500 nm, where kappa is the ratio ofdispersion in the LP₀₂ mode at 1550 nm divided by dispersion slope inthe LP₀₂ mode at 1550 nm.
 9. The waveguide of claim 8 wherein the kappavalue is in the range between about 30 nm and about 70 nm.
 10. Thewaveguide of claim 1 wherein the refractive index profile is selected toresult in the waveguide exhibiting an effective area greater than about30 μm² at about 1550 nm in the LP₀₂ mode.
 11. The waveguide of claim 10wherein the refractive index profile is selected to result in thewaveguide exhibiting an effective area greater than about 60 μm². 12.The waveguide of claim 11 having a length between about 0.5 km and about3 km and is capable of propagating a light signal in an LP₀₂ mode alongan entire length thereof to compensate for dispersion of a length oftransmission fiber adapted to propagate the light signal in an LP₀₁mode.
 13. The waveguide of claim 10 wherein the refractive index profileis selected to result in the waveguide exhibiting an effective area inthe range between about 30 μm² and 150 μm².
 14. The waveguide of claim10 wherein the refractive index profile is selected to result in thewaveguide exhibiting an effective area in the range between about 50 μm²and 90 μm².
 15. The waveguide of claim 1 wherein the refractive indexprofile is selected to result in the waveguide exhibiting a dispersionvalue at about 1550 nm and in the LP₀₂ mode between about −10 ps/nm/kmand about −1000 ps/nm/km.
 16. The waveguide of claim 15 wherein therefractive index profile is selected to result in the waveguideexhibiting a dispersion value at about 1550 nm and in the LP₀₂ modebetween about −50 ps/nm/km and about −400 ps/nm/km.
 17. The waveguide ofclaim 1 wherein the refractive index profile is selected to result inthe waveguide exhibiting a dispersion slope value at about 1550 nm andin the LP₀₂ mode between about −0.01 ps/nm²/km and about −20 ps/nm²/km.18. The waveguide of claim 17 wherein the refractive index profile isselected to result in the waveguide exhibiting a dispersion slope valueat about 1550 nm and in the LP₀₂ mode between about −1 ps/nm²/km andabout −10 ps/nm²/km.
 19. The waveguide of claim 1 wherein the refractiveindex profile is selected to result in the waveguide exhibiting: (a) adispersion value at about 1550 nm and in the LP₀₂ mode between about −50ps/nm/km and about −400 ps/nm/km. (b) a dispersion slope value at about1550 nm and in the LP₀₂ mode between about −1 ps/nm²/km and about −10ps/nm²/km, and (c) an effective area in the range between about 50 μm²and 90 μm² at about 1550 nm in the LP₀₂ mode.
 20. The waveguide of claim1 wherein the plurality of core segments comprises at least three coresegments.
 21. The waveguide of claim 1 wherein n₁>n₃>n₂.
 22. Thewaveguide of claim 1 wherein a first core segment of the plurality ofcore segments comprises a Δ% peak in the range between about 1.0% and2.5%.
 23. The waveguide of claim 1 wherein a first core segment of theplurality of core segments comprises a Δ% peak greater than about 1.5%.24. The waveguide of claim 23 wherein the Δ% peak is greater than about2.0%.
 25. The waveguide of claim 1 wherein a second core segment of theplurality of core segments comprises a Δ% peak of greater than about0.0%.
 26. The waveguide of claim 1 wherein the Δ% peak of the secondcore segment is in the range between about 0.3% and about −0.1%.
 27. Thewaveguide of claim 1 wherein a third core segment of the plurality ofcore segments comprises a Δ% peak in the range between about 0.2% andabout 1.0%.
 28. The waveguide of claim 27 wherein the Δ% peak of thethird core segment is in the range between about 0.3% and about 0.6%.29. The waveguide of claim 1 wherein a second core segment of theplurality of segments has a radius R3 between about 10 μm and about 20μm.
 30. The waveguide of claim 29 further comprising a radius R2 betweenabout 7 μm and about 13 μm.
 31. The waveguide of claim 1 furthercomprising a first core segment of the plurality of core segmentsincluding a Δ% of less than about 1.0 at a centerline of the dispersioncompensating waveguide and a Δ% peak at a radius greater than 1 μmhaving a Δ% peak of greater than about 1.5%.
 32. The waveguide of claim1 wherein a first core segment of the plurality of core segments has apeak Δ% in the range of between about 1.0% and about 2.5% and ispositioned at a radius dimension between about 1 μm and about 3 μm. 33.The waveguide of claim 1 wherein the dispersion compensating opticalwaveguide has a length between about 0.5 km and about 3 km.
 34. Thewaveguide of claim 1 having a length between about 0.5 km and about 3.0km.
 35. A dispersion compensating optical waveguide comprising: aplurality of core segments, the refractive index profile of whichresults in a waveguide capable of propagating a light signal in an LP₀₂mode at about 1550 nm by the dispersion compensation optical waveguidethrough a sufficient length, upon conversion to the LP₀₂ mode, tocompensate for dispersion of a transmission waveguide propagating in aLP₀₁ mode at about 1550 nm, the transmission waveguide having a lengthgreater than about 25 km, the dispersion compensating optical waveguideincluding; (a) a first core segment having an outer radius in the rangebetween about 4 μm and 8 μm and a Δ% peak of greater than 1.5%, (b) asecond core segment having an outer radius in the range between about 8μm and 12 μm and a Δ% peak greater than 0.0%, and (c) a third coresegment having an outer radius between about 10 μm and 25 μm and a Δ%peak greater than about 0.2%.
 36. A dispersion compensating waveguidecomprising: a plurality of core segments, the refractive index profileof which results in properties capable of propagating a light signal inan LP₀₂ mode at about 1550 nm by the dispersion compensation waveguidethrough a sufficient length, upon conversion to the LP₀₂ mode and at1550 nm, to compensate for dispersion of a transmission waveguidepropagating in a LP₀₁ mode, the transmission waveguide having a lengthgreater than 25 km, the dispersion compensating optical waveguideincluding; (a) a first core segment having an outer radius in the rangebetween about 4 μm and 8 μm and a Δ% peak in the range between aboutgreater than 1.5%, (b) a sub-segment within the first core segmenthaving a inner radius of between about 3 μm and 6 μm and a Δ% peak inthe range between about 0.6% and about 1.4%, (c) a second core segmenthaving an outer radius in the range between about 8 μm and 12 μm and aΔ% peak in the range between about 0.3% and about −0.5%, and (d) a thirdcore segment having an outer radius between about 14 μm and 20 μm and aΔ% peak in the range between about 0.2% and about 0.8%.
 37. A dispersioncompensating waveguide comprising: a plurality of core segments, therefractive index profile of which is selected to result in thedispersion compensating waveguide being capable of propagating a lightsignal in an LP₀₂ mode at about 1550 nm a sufficient distance tocompensate for dispersion of a transmission waveguide having a lengthgreater than 25 km that propagates the light signal in an LP₀₁ mode atabout 1550 nm, the dispersion compensating waveguide having thefollowing properties: (a) a kappa value between about 30 nm to about 150nm at about 1550 nm and in the LP₀₂ mode; (b) an effective area betweenabout 30 μm² and about 150 μm² at about 1550 nm and in the LP₀₂ mode;and (c) a dispersion value of −50 to −400 ps/km/nm at about 1550 nm andin the LP₀₂ mode.
 38. A dispersion compensating module comprising: acoupler adapted to couple with a first fiber designed to propagate lightin a first mode, a reflective fiber grating operatively connected to thecoupler, the fiber grating capable of converting the first mode into asecond mode, and a second fiber operatively coupled through the couplerto the reflective fiber grating, the second fiber capable of propagatinglight in the second mode.
 39. The module of claim 38 wherein the firstfiber is a transmission fiber and the second fiber is a dispersioncompensating fiber.
 40. The module of claim 38 wherein the second fibercomprises: a plurality of core segments, the refractive index profile ofwhich results in a waveguide capable of propagating a light signal in anLP₀₂ mode at about 1550 nm by the dispersion compensation waveguidethrough a sufficient length, upon conversion to the LP₀₂ mode and at1550 nm, to compensate for dispersion of a transmission waveguidepropagating in a LP₀₁ mode, the transmission waveguide having a lengthgreater than 25 km, the dispersion compensating optical waveguideincluding; (a) a first core segment having an outer radius in the rangebetween about 4 μm and 8 μm and a Δ% peak in the range between aboutgreater than 1.5%, (b) a sub-segment within the first core segmenthaving an inner radius of between about 3 μm and 6 μm and a Δ% peak inthe range between about 0.6% and about 1.4%, (c) a second core segmenthaving an outer radius in the range between about 8 μm and 12 μm and aΔ% peak in the range between about 0.3% and about −0.5%, and (d) a thirdcore segment having an outer radius between about 14 μm and 20 μm and aΔ% peak in the range between about 0.2% and about 0.8%.
 41. The moduleof claim 38 wherein the first mode is an LP₀₁ mode and the second modeis an LP₀₂ mode.
 42. The module of claim 38 wherein the coupleroptically couples a pigtail to the reflective fiber grating andoptically couples the reflective fiber grating to a fiber interconnect.43. The module of claim 38 further comprising a fiber interconnect whichoptically couples the fiber grating to a dispersion compensating fiber.44. The module of claim 43 wherein the dispersion compensating fiberincludes a refractive index profile, a core portion of whichsubstantially matches the shape of the refractive index profile of acore portion of the fiber interconnect.
 45. The module of claim 38further comprising a pass through fiber including the reflective fibergrating on one portion thereof and a fiber interconnect on anotherportion thereof.
 46. The module of claim 45 wherein the pass throughfiber includes a necked-down portion wherein a width dimension of aglass fiber portion of the pass through fiber is reduced as compared toan initial undeformed dimension thereof, the necked down portion beingformed prior to fusion of the first and second fibers within thecoupler.
 47. The module of claim 46 wherein a transverse dimension ofthe necked-down portion is pre-selected such that a propagation constantof the first fiber substantially matches a propagation constant of thesecond fiber at an operating mode.
 48. The module of claim 38 whereinthe reflective fiber grating is included on a conversion fiber thatincludes boron doped silica.
 49. The module of claim 48 furthercomprising germanium doped silica.
 50. The module of claim 48 furthercomprising phosphorous doped silica.
 51. The module of claim 38 whereinthe reflective fiber grating is included on a conversion fiber having afirst core segment including inner and outer sub-segments, the innersub-segment having a first peak Δ% and includes germanium doped silica,the outer sub-segment having a lower Δ% than the inner sub-segment andincludes phosphorous doped silica.
 52. The module of claim 38 whereinthe reflective fiber grating is included on a conversion fiber having afirst core segment including inner and outer sub-segments, the innersub-segment having a first peak Δ% and including boron and germaniumdoped silica, the outer sub-segment having a second peak Δ% lower thanthe first peak Δ% and wherein the outer sub-segment includes phosphorousdoped silica.
 53. A dispersion compensating module comprising: (a) amode converter operatively coupleable with a transmission waveguide, thetransmission waveguide adapted to propagate light in a first mode, themode converter including a reflective fiber grating capable ofconverting the first mode into a second mode, and (b) a dispersioncompensating fiber operatively coupled to the mode converter, thedispersion compensating fiber adapted to propagate light in the secondmode such that dispersion of the transmission fiber may be compensatedfor.
 54. The module of claim 53 further comprising an optical fibercoupler adapted to couple light propagating in the first mode into thereflective fiber grating and further adapted to couple light propagatingin the second mode into the dispersion compensating fiber.
 55. Themodule of claim 53 wherein the first mode is a LP₀₁ mode and the secondmode is an LP₀₂ mode.
 56. A mode converter comprising: (a) an opticalfiber coupler adapted to operatively couple light propagating in a firstmode in a first fiber into a second fiber, and (b) a reflective fibergrating operatively coupled to the second fiber, the grating beingcapable of converting light propagating in a first mode into a secondmode wherein the second fiber extends from the optical fiber coupler andis adapted to propagate light in the second mode.
 57. The converter ofclaim 56 wherein the first fiber is a fiber pigtail adapted tooperatively couple to an optical transmission waveguide; thetransmission waveguide propagating light in an LP₀₁ mode.
 58. Theconverter of claim 56 wherein the reflective fiber grating is adapted toconvert the LP₀₁ mode into an LP₀₂ mode.
 59. The converter of claim 56wherein the second fiber is a fiber interconnect operatively coupled tothe reflective fiber grating, the fiber interconnect being adapted tocouple with a dispersion compensating fiber adapted to propagate lightin the LP₀₂ mode.
 60. The mode converter of claim 56 wherein the coupleris adapted to operatively couple a pigtail with the reflective fibergrating and the fiber grating with a fiber interconnect.
 61. The modeconverter of claim 56 wherein the reflective fiber grating comprises aplurality of longitudinally spaced portions that have been exposed to UVradiation to change a refractive index of the portions.
 62. The modeconverter of claim 61 wherein the longitudinally spaced portions arespaced at intervals that vary by up to 3% from a beginning to an end ofthe reflective fiber grating.
 63. The mode converter of claim 56 whereina fiber having the reflective grating thereon includes: a first coresegment having an outer radius in the range between about 3 μm and about7 μm and a Δ% peak greater than about 1.2%, and a outer sub-segmentwithin the first core segment having an inner radius of between about 2μm and about 5 μm and a Δ% peak in the range between about 0.4% andabout 1.2%.
 64. The module of claim 56 wherein the reflective fibergrating is included in a fiber that comprises boron doped silica in afirst segment thereof.
 65. The module of claim 64 further comprisinggermanium doped silica in a first segment thereof.
 66. The module ofclaim 65 further comprising phosphorous doped silica.
 67. The module ofclaim 56 wherein the reflective fiber grating is included in a fibercomprising germanium and boron doped silica in a first portion andphosphorous doped silica in a second portion thereof.
 68. A opticalfiber coupler comprising: (a) a first optical fiber within the couplerhaving a first propagation constant in a first mode, and (b) a secondoptical fiber within the coupler, the second optical fiber having asecond propagation constant in an undeformed portion thereof and in thefirst mode that is different than the first propagation constant, thesecond optical fiber including a necked-down portion formed on a glassportion thereof which is formed prior to fusion of the fibers, thenecked-down portion having a dimension such that a propagation constantin the necked-down portion substantially matches the first propagationconstant wherein coupling of light between the fibers in the first modeis enhanced.
 69. The coupler of claim 68 further comprising a canesleeve into which the fibers are inserted, the sleeve and fibers beingfused together at a mid-region of the sleeve.
 70. The coupler of claim69 wherein the cane sleeve comprises boron doped silica.
 71. The couplerof claim 70 wherein boron is included in an amount of up to 10% byweight of silica.
 72. The coupler of claim 68 wherein the first opticalfiber is a single mode fiber.
 73. The coupler of claim 68 wherein thesecond optical fiber includes a reflective fiber grating.
 74. Thecoupler of claim 68 wherein the second optical fiber is adapted topropagate a higher order optical mode.
 75. The coupler of claim 74wherein the higher order optical mode is an LP₀₂ mode.
 76. The couplerof claim 68 wherein the necked-down portion is about 30% to about 60% ofan undeformed transverse dimension of the second optical fiber.
 77. Aoptical fiber coupler, comprising: (a) a cane sleeve having a length, amid-region, opposed ends and an aperture therethrough, (b) a first fiberadapted for carrying a light in a first mode received within theaperture of the cane sleeve and extending out of at least one of theopposed ends, and (c) a second fiber passing through the sleeve andextending out of both ends, the second fiber adapted for propagatinglight in a second mode and including a necked-down portion formed on aglass portion of the second fiber prior to fusion of the fibers, thenecked-down portion having a length less than the length of the sleeveand being positioned approximately at the mid-region, the first fiber,second fiber and sleeve being fused and stretched under heat along themid-region such that upon light transmission through one of the fibers,a portion of light is coupled into the other fiber.